Ink may be used to make aesthetically pleasing designs on consumer products to increase sales of the consumer products. Applying ink to a substrate, such as a paper web, is well known in the art.
A known apparatus for applying ink to a substrate comprises an ink fountain, or reservoir, and a cylindrical roller rotatably mounted on a frame. The roller is partially submerged in the ink in the reservoir. The reservoir may be open to the atmosphere or, alternatively, may be closed and pressurized.
As the roller is rotated, the surface of the roller carries ink from the ink reservoir for subsequent transfer to the substrate. The partially submerged roller can be an anilox roller having an engraved surface with cells to enhance transfer of the ink from the reservoir. The anilox roller can form a nip with a plate cylinder. Ink transferred from the anilox roller to the plate cylinder is applied to a substrate passing between the plate cylinder and an impression cylinder.
Such a printing apparatus is disclosed in allowed U.S. patent application Ser. No. 07/917,528, Issue Batch No. J48, Apparatus for Applying Ink to a Substrate, filed Jul. 17, 1992 in the name of Leopardi, which application is incorporated by reference for the purpose of showing an ink reservoir and anilox roller assembly.
Generally, it is desirable to operate the printing apparatus at high speeds to decrease the unit cost of the printed substrate and provide a competitively priced product. However, attempts to increase the operating speed of the printing apparatus can result in undesirable variations in ink transfer to the anilox roller. Variations in ink transfer from the reservoir to the anilox roller can result in a low quality appearance of the ink patterns on the paper substrate due to concomitant variations in definition and intensity of the pattern.
Such variations are believed to be caused, at least in part, by energy and air directed into the ink in the reservoir by the anilox roller. As the anilox roller is rotated in the reservoir, energy is transferred from the surface of the anilox roller to the ink in the reservoir. The energy transferred to the ink increases as the rotational speed of the anilox roller increases.
The energy transferred to the ink can result in large scale turbulent flow of the ink within the reservoir. Ink flow caused by the rotation of the anilox roller is initially primarily in the machine direction which corresponds to the direction of rotation of the anilox roller. This ink flow can be turned by the walls of the reservoir, such that the large scale turbulent flow of the ink is directed along the length of the anilox roller in the cross machine direction. This cross machine direction is generally parallel to the axis of rotation of the anilox roller.
The rotating anilox roller can also carry a boundary layer of air on its surface. This boundary layer of air can be carried into the ink in the reservoir, especially if the reservoir is open to the atmosphere. The amount of air entrained in the ink also increases with the rotational speed of the anilox roller. The entrained air can result in undesirable foaming of the ink and prevent ink from entering the cells on the surface of the anilox roller.
The entrained air, in combination with the turbulence in the ink, can result in the formation of traveling ink waves along the length of the anilox roller. The troughs of the waves can cause uneven inking along the length of the anilox roller which in turn causes undesirable variations in the pattern on the inked substrate.
One known method of dissipating the flow energy of the ink caused by the anilox roller provides a series of parallel walls or plates positioned in the reservoir. Examples of plate baffles are disclosed in U.S. Pat. No. 2,276,662 issued Mar. 17, 1942, to Matuschke, U.S. Pat. No. 4,138,333 issued Jun. 19, 1979, to Navi, U.S. Pat. No. 4,373,443 issued Feb. 15, 1983, to Matalia et al., and U.S. Pat. No. 4,497,250 issued Feb. 5, 1985, to Dressler. Such plate baffles are disadvantageous because they divide the reservoir into compartments, and can thereby prevent proper mixing or circulation of the ink within the reservoir. Poor ink circulation can cause undesirable changes in ink properties, such as increased ink viscosity, which can result in variations in the intensity of the pattern printed on the substrate.
Further, plate baffles can prevent ink flow in a direction perpendicular to the plates, while permitting unrestricted flow parallel to the plates. Therefore, there is little or no dissipation of ink flow energy directed parallel to the plates.
Another known method of dissipating the flow energy of the ink caused by the anilox roller comprises filling the reservoir with a number of pads. Each pad comprises a three dimensional mesh of one or more plastic strands. An example of such a mesh is a scrubbing pad, such as is available from the Miles Corporation of Chicago, Illinois as Tuffy Dishwashing Pads. The plastic mesh pads can simply be placed in the reservoir, or can be supported in a mesh cage having a periphery that extends the length, width, and depth of the reservoir.
The above mentioned pads are effective in breaking up the flow of ink caused by the rotations of the anilox roller. However, the pads can prevent sufficient circulation of the ink within the reservoir, which can result in an undesirable change in ink properties, and variations in print intensity on the substrate. Insufficient circulation is believed to be due, at least in part, to the dense construction of the pads. The pads have a surface area to volume ratio of about 5.9 square centimeters per cubic centimeter (15 square inches per cubic inch), where the surface area is the surface area of the plastic strands and the volume is the volume enclosed by the periphery of the pad. This volume includes the volume occupied by the plastic strands and the volume of the ink between the strands.
The surface area to volume ratio provides one measure of damping in a baffle. Flow energy is dissipated as the ink flows over a surface, so that flow energy dissipation can be increased by increasing the surface area to volume ratio of the baffle. This surface area to volume ratio and restriction to flow can increase if the pads are compressed as they are loaded into the reservoir. Therefore, the spacing between strands (and the resulting restriction to flow through the pads) can vary depending on the number of pads placed in the reservoir and the manner in which they are packed in the reservoir.
The dense structure of the pads also results in the pads becoming clogged with paper fibers or other impurities in the ink, which clogging further reduces ink circulation. The pads must therefore be frequently cleaned or replaced. Each time the pads must be cleaned or replaced the printing apparatus must be shut down, resulting in lost production.
The pads are al so disadvantageous because they provide a restriction to flow which is substantially the same in any direction. An ink reservoir may have an ink inlet in the bottom of the reservoir. In such a reservoir, relatively unrestricted vertical flow from the inlet to the anilox roller is desirable, while restricted flows in the machine direction and cross machine direction are desirable in order to dissipate the flow energy of the ink caused by the rotation of the anilox roller.
Accordingly, it is an object of the present invention to provide a printing apparatus having a baffle with flow restrictors that dissipate flow energy in an ink reservoir while permitting circulation of the ink within the ink reservoir.
A further object of the present invention is to provide a baffle that prevents direct flow through the baffle in a machine direction and in a cross machine direction, while permitting circulation between flow restrictors in the machine and cross machine directions. Yet a further object of the present invention is to provide a baffle that restricts flow in the machine and cross machine directions substantially more than in a vertical direction normal to the machine and cross machine directions.